Precise pre-deployment measurement of physical parameters, such as temperature and heat flow, under dynamic loads in satellite or other testing often requires adding large numbers of wires to support thermocouples or other types of probes. Not only do the wires provide a thermal conduction path that disturbs the measurement, but the wires also change the dynamics of the body during acceleration and shock testing due to their added mass. Therefore, wireless sensors are attractive for such physical measurements.
Surface acoustic wave (SAW) devices have been demonstrated to function linearly as temperature sensors over a broad range of temperature and they have been employed as wireless tags for tracking of high value assets in addition to various types of wireless sensors. See M. Viens and J. D. N. Cheeke, “Highly Sensitive Temperature Sensor Using SAW Resonator Oscillator,” Sensors and Actuators A 24, 209 (1990); L. Mingfang and L. Haiguo, “SAW temperature and humidity sensor with high resolution,” Sensors and Actuators B 12, 53 (1993); J. D. Sternhagen et al., “A Novel Integrated Acoustic Gas and Temperature Sensor,” IEEE Sensors Journal 2 (4), 301 (2002); G. Scholl et al., “Surface Acoustic Wave Devices for Sensor Applications”, Phys. Stat. Sol. A 185 (1), 47 (2001); and A. von Jena et al., “Intelligent Sensor for Monitoring Freight-Waggon Working Conditions,” Sensors and Actuators A 42, 347 (1994). Wireless tagging and sensing applications require a method of self-identification, such as a unique photo-lithographically patterned bit sequence of reflection structures, to be fabricated on the device during manufacture. These bit sequences allow a unique identification code to be hardwired into a SAW device, enabling identification of the particular part. This bit sequence requires each part on the wafer to have a unique photo-mask and thus increases fabrication cost during dicing and packaging due to tracking requirements on that unique part. In addition, some of the energy received by the SAW device via the antenna is used for the identification and thus reduces the energy available for the sensing measurement. This loss of energy reduces the signal-to-noise ratio of the SAW temperature sensor.
For application as a sensor, there must be some environmentally introduced change in the wireless SAW response that can be detected by a readout device. An example of this type of sensor is described in the literature where the phase angle of a reflected signal is monitored as function of pressure or temperature using a network analyzer. See G. Scholl et al. This type of sensor response is good for monitoring pressure, strain, and torque but is limited by the expense and portability of the readout hardware. Further, SAW devices typically operate at frequencies in the range of 100 MHz to several GHz, making the monitoring of the phase and individual waves impractical for portable, inexpensive systems.
Therefore, a need remains for a wireless passive temperature sensor that can remotely measure the thermal profile and heat flow of a structure using a portable, inexpensive transceiver.